Field
Aspects of the present disclosure relate generally to solid-states lighting devices, and more particularly, to a light-emitting device with near full spectrum light output.
Background
Light emitting diodes (LEDs) are monochromatic solid-state lighting devices that emit light in one color. Before the advent of blue LEDs, which provided light in a bluish yellow color spectrum through excitation of a phosphor powder in the blue LED semiconductor structure, color output from LEDs were mainly limited to various hues of red, green, and yellow. Using blue LEDs, “light bulbs” may be made by using one LED or by grouping two or more blue LEDs to provide enough light output in an LED bulb. Although light emitted from blue LEDs more closely approaches the appearance of white light, the blue and yellow light produced by these blue LEDs still possesses poor color characteristics, as measured by a quantitative scale defined by the International Commission on Illumination (CIE), based in France, known as color rendering index (CRI). CRI is a quantitative measure of how well a light source is able to illuminate various objects in comparison with an ideal or natural light source to faithfully reproduce the colors of the objects. Thus, light sources with a high CRI are desirable, and their use is a necessity in color-critical applications such as photography and cinematography. In contrast, light sources with low CRI, such as those described above implemented with blue LEDs, are not preferable light sources for everyday use.
It is not uncommon in the lighting industry to find LED bulbs that output a bluish light. As white light is composed of several colors (i.e., white light may only be produced with a mixture of two and more colors), these bulbs will never produce light with desirable characteristics (as measured using CRI) without any source of light in the red spectrum in the light output from these LED bulbs. Because LED bulbs with light output having higher CRIs are more desirable, many LED bulbs also include one or more red LED chips with the one or more blue LEDs in forming the groups of LEDs in each LED bulb. These red LEDs, which emit red light at a wavelength between 600 to 630 nm, introduce a higher value of R9 and hence, a higher CRI can be obtained. The achievement of higher CRI through this approach, however, incurs additional negative effects. For example, color drift of LED chips is related to the temperature of the LED chip, and at high temperatures this drift is quite significant. It should be apparent that a color drift of any of the LEDs in an LED bulb would cause a color drift of the LED bulb. To address this issue, some approaches add a separate electronic driver circuit to the LED bulb to provide some control over the operation of the red LEDs. In addition to increasing complexity of the drive circuit for the LED bulb as a whole, the inclusion of the additional circuitry also increases the cost of each LED bulb and introduces a higher failure rate during production. U.S. Pat. No. 7,213,940 provides that 2 groups of blue and red LEDs with light output in the ranges of 430 nm to 480 nm and 600 nm to 630 nm, respectively, together produces a mixture of light having x, y color coordinates with an area on a 1931 CIE Chromaticity Diagram defined by points having coordinates (0.32, 0.40), (0.36, 0.48), (0.43, 0.45), (0.42, 0.42), (0.36, 0.38). When electric current passes through these LEDs, the LEDs will generate heat and this changes the operating temperature of the blue and red LEDs. Changes in LEDs operating temperature make the color emitted from the LEDs unstable. In addition, because color change of red LEDs due to temperature change occurs at a different rate than blue LEDs, the rate of change in the emitted color from the 2 groups of LEDs itself is unstable. To stabilize the color characteristics in the light that is output, two different electronic LED drive circuits are used to separately control the blue and red LEDs.
On-going demands for improvement to LED technology continue to increase due to the high efficiency possessed by, and varied applications for, these devices. For example, as demand for LED lighting products such as LED bulbs increase, need for more efficient and less-complex solutions to achieve emission of white light with LED bulbs will also increase commensurately.